1. Field of the Invention
This invention relates to weighing apparatus and, more particularly, to weighing apparatus utilizing multiple load cells and RF (radio frequency) to communicate,
2. Description of Prior Art
Many weighing applications require the use of multiple load cells in a single scale or in a number of associated scales. For example, a heavy capacity scale for weighing trucks or railroad cars requires multiple load cells. Each load cell provides an analog signal proportional to the portion of the load borne by that load cell. Strain gages connected in a wheatstone bridge configuration often provide the analog signal. In heavy capacity applications, the load is distributed over usually at least four load cells and some applications may require sixteen or more load cells. The sum of the load cell output signals must be obtained to provide a signal representative of the total weight applied to the scale.
The weight accuracy of multiple load cell scales depends not only on the accuracy of the individual cells, but also on the mechanical and electrical interection among them. Since the load cells usually have different sensitivities to applied loads the total scale output is usually dependent upon the position of the weight on the scale. The outputs of the individual cells must therefore be compensated or adjusted so that the total scale output remains substantially the same for a given load no matter where on the scale it is positioned. Such load position compensation has usually been accomplished by connecting sensitivity reducing resistors in the wheatstone bridge circuit of the individual load cells, usually across the output of the bridge circuit. U.S. Pat. Nos. 4,261,195 to Lockery, 4,574,899 to Griffen, and 4,556,115 to Lockery address the problem of load position compensation in multi-load cell scales.
A large capacity weigh scale usually consists of multiple load cells. Typically a rocker pin, guided beam, torsion ring type load cell is used. U.S. Pat. No. 4,815,547 to Griffen, et al. discloses a typical rocker pin with a printed circuit board attached. A principal advantage of the rocker pin is that it can be made self-erecting, that is, so that when the normally upright pin is deflected about its base or grounded end, the pin will return to its upright position when the deflecting load is removed. The self-erecting feature is an advantage in weighing applications in which temporary side loads are encountered. The self-erecting feature is obtained by configuring the pin so that the radius of curvature of each end surface is greater than half the total height of the pin.
In the present art, there is a need for anti-rotation methods for the load cells, as the rotation of the load cell would produce wear on the load cell and stress on the cables. One methods that are used is the use of hexes on the heads of the load cell pins. Hexes are used so that the load cell will not rotate. Since the load cell cannot rotate, the wear on the load cell is not distributed evenly.
Recently, there has appeared the so-called “digital load cell” in which an analog-to-digital converter and microprocessor are dedicated to a single load cell. The electronic circuits are mounted on a printed circuit board connected directly to the counterforce. This development has permitted digital correction of various load cell inaccuracies
When the analog circuits of the load cells are connected together, they are essentially impossible to monitor individually. Thus, “trouble shooting” or repair of a scale, can require disassembly of the electrical circuits in order to test the load cells individually and find the defective one. Further, when a load cell is replaced for any reason, the scale often requires recompensation for load position. A known test weight is required to accomplish this recompensation. For large scales in particular this is a time consuming procedure, and the known weight is often inconvenient to obtain. U.S. Pat. No. 4,804,052 to Griffen discloses such a compensated multiple load cell.
The current art of large capacity weigh scales has armored cables to provide the electrical interaction between the individual load cells and the controller. There are also cables that run from the master controller to the control house. The cables are expensive and subject to being damaged, being pinched, being eaten by rodents, and failing. The cables tend to break down due to weather changes. This requires the scale to be repaired at heavy cost. This also means that the scale can be down for long periods of time. In the present art, if all cables are inter-connected and if a cable breaks in one spot, the whole system could be dead. There is a large risk to the scale due to lightening damage because of the wiring in the scale. There is also risk of power interruption to the large capacity scale, which would shut down the ability to use the scale. If the power cable to the controller is down, then the whole scale will be down. There is also a limitation on the control house based on the need to run cables from the control house to the scale. The terrain and distance must be taken into account.
There is still room for improvement within the art.
1. Field of the Invention
U.S. Class 177-25.14
2. Description of Related Art Including Information Disclosed Under 37 CFR § 1.97**>and 1.98<.